Engineers have developed a method that allows pre-designed sequences of DNA to autonomously grow and concatenate along specific assembly routes, hence providing the basis for a new generation of programmable molecular devices. Putting their new concept of so-called 'Primer Exchange Reaction' (PER) cascades to the test, they successfully engineered a first set of devices with diverse functions, such as self-building DNA-origami and DNA nanostructures that sense, amplify, record or logically evaluate environmental signals.